Method and apparatus for creating a bi-polar virtual electrode used for the ablation of tissue

ABSTRACT

A method and apparatus for creating a virtual electrode to ablate bodily tissue. The apparatus includes an outer tube, a first electrode, an inner tube and a second electrode. The outer tube is fluidly connected to a source of conductive fluid and defines a proximal end and a distal end. The distal end includes an opening for delivering conductive fluid from the outer tube. The first electrode is disposed at the distal end of the outer tube for applying a current to conductive fluid delivered from the outer tube. The inner tube is coaxially received within the outer tube and is connected to a source of conductive fluid. The inner tube defines a proximal end and a distal end, with the distal end forming an opening for delivering conductive fluid from the inner tube. Finally, the second electrode is disposed at the distal end of the inner tube for applying a current to conductive fluid delivered from the inner tube. With this configuration, upon final assembly, the distal end of the outer tube is axially spaced from the distal end of the inner tube such that the first electrode is spaced from the second electrode. As a result, a bi-polar virtual electrode can be established.

This application claims the benefit of U.S. Provisional Application No.60/091,929, filed on Jul. 7, 1998.

FIELD OF THE INVENTION

The present invention relates generally to an apparatus for creating avirtual electrode. More particularly, the present invention relates toan apparatus for the creation of a virtual electrode that is useful forthe ablation of soft tissue and neoplasms.

BACKGROUND OF THE PRESENT INVENTION

The utilization of an electric current to produce an ameliorative effecton a bodily tissue has a long history, reportedly extending back to theancient Greeks. The effects on bodily tissue from an applied electriccurrent, and thus the dividing line between harmful and curativeeffects, will vary depending upon the voltage levels, current levels,the length of time the current is applied, and the tissue involved. Onesuch effect resulting from the passage of an electric current throughtissue is heat generation.

Body tissue, like all non-superconducting materials, conducts currentwith some degree of resistance. This resistance creates localizedheating of the tissue through which the current is being conducted. Theamount of heat generated will vary with the power P deposited in thetissue, which is a function of the product of the square of the currentI and the resistance R of the tissue to the passage of the currentthrough it (P=I²R.).

As current is applied to tissue, then, heat is generated due to theinherent resistance of the tissue. Deleterious effects in the cellsmaking up the tissue begin to occur at about 42° Celsius. As thetemperature of the tissue increases due to heat generated by thetissue□s resistance, the tissue will undergo profound changes andeventually, as the temperature becomes high enough, that is, generallygreater than 45° C., the cells will die. The zone of cell death is knownas a lesion and the procedure followed to create the lesion is commonlycalled an ablation. As the temperature increases beyond cell deathtemperature, complete disintegration of the cell walls and cells causedby boiling off of the tissue's water can occur. Cell death temperaturescan vary somewhat with the type of tissue to which the power is beingapplied, but generally will begin to occur within the range of 45° to60° C., though actual cell death of certain tissue cells may occur at ahigher temperature.

In recent times, electric current has found advantageous use in surgery,with the development of a variety of surgical instruments for cuttingtissue or for coagulating blood. Still more recently, the use ofalternating electric current to ablate, that is, kill, various tissueshas been explored. Typically, current having a frequency from about 3kilohertz to about 300 gigahertz, which is generally known asradiofrequency or radiofrequency (RF) current, is used for thisprocedure. Destruction, that is, killing, of tissue using an RF currentis commonly known as radiofrequency ablation. Often radiofrequencyablation is performed as a minimally invasive procedure and is thusknown as radiofrequency catheter ablation because the procedure isperformed through and with the use of a catheter. By way of example,radiofrequency catheter ablation has been used to ablate cardiac tissueresponsible for irregular heart beats or arrythmias.

The prior art applications of current to tissue have typically involvedapplying the current using a dry electrode. That is, a metal electrodeis applied to the tissue desired to be affected and a generated electriccurrent is passed through the electrode to the tissue. A commonly knownexample of an instrument having such an operating characteristic is anelectrosurgical instrument known as a bovie knife. This instrumentincludes a cutting/coagulating blade electrically attached to a currentgenerator. The blade is applied to the tissue of a patient and thecurrent passes through the blade into the tissue and through thepatients body to a metal base electrode or ground plate usually placedunderneath and in electrical contact with the patient. The baseelectrode is in turn electrically connected to the current generator soas to provide a complete circuit.

As the current from the bovie knife passes from the blade into thetissue, the resistance provided by the tissue creates heat. In thecutting mode, a sufficient application of power through the bovie knifeto the tissue causes the fluid within the cell to turn to steam,creating a sufficient overpressure so as to burst the cell walls. Thecells then dry up, desiccate, and carbonize, resulting in localizedshrinking and an opening in the tissue. Alternatively, the bovie knifecan be applied to bleeding vessels to heat and coagulate the bloodflowing therefrom and thus stop the bleeding.

As previously noted, another use for electrical instruments in thetreatment of the body is in the ablation of tissue. To expand further onthe brief description given earlier of the ablation of cardiac tissue,it has long been known that a certain kind of heart tissue known assino-atrial and atrio-ventricular nodes spontaneously generate anelectrical signal that is propagated throughout the heart alongconductive pathways to cause it to beat. Occasionally, certain hearttissue will misfire, causing the heart to beat irregularly. If theerrant electrical pathways can be determined, the tissue pathways can beablated and the irregular heartbeat remedied. In such a procedure, anelectrode is placed via a catheter into contact with the tissue and thencurrent is applied to the tissue via the electrode from a generator ofRF current. The applied current will cause the tissue in contact withthe electrode to heat. Power will continue to be applied until thetissue reaches a temperature where the heart tissue dies, therebydestroying the errant electrical pathway and the cause of the irregularheartbeat.

Another procedure using RF ablation is transurethral needle ablation, orTUNA, which is used to create a lesion in the prostate gland for thetreatment of benign prostatic hypertrophy (BPH) or the enlargement ofthe prostate gland. In a TUNA procedure, a needle having an exposedconductive tip is inserted into the prostate gland and current isapplied to the prostate gland via the needle. As noted previously, thetissue of the prostate gland heats locally surrounding the needle tip asthe current passes from the needle to the base electrode. A lesion iscreated as the tissue heats and the destroyed cells may be reabsorbed bythe body, infiltrated with scar tissue, or just become non-functional.

While there are advantages and uses for such dry electrode instruments,there are also several notable disadvantages. One of these disadvantagesis that during a procedure, coagulum-dried blood cells and tissuecells—will form on the electrode engaging the tissue. Coagulum acts asan insulator and effectively functions to prevent current transfer fromthe blade to the tissue. This coagulum insulation can be overcome withmore voltage so as to keep the current flowing, but only at the risk ofarcing and injuring the patient. Thus, during surgery when the tissue iscut with an electrosurgical scalpel, a build-up of coagulated blood anddesiccated tissue will occur on the blade, requiring the blade to becleaned before further use. Typically, cleaning an electrode/scalpelused in this manner will involve simply scraping the dried tissue fromthe electrode/scalpel by rubbing the scalpel across an abrasive pad toremove the coagulum. This is a tedious procedure for the surgeon and theoperating staff since it requires the real work of the surgery to bediscontinued while the cleaning operation occurs. This procedure can beavoided with the use of specially coated blades that resist the build upof coagulum. Such specialty blades are costly, however.

A second disadvantage of the dry electrode approach is that theelectrical heating of the tissue creates smoke that is now known toinclude cancer-causing agents. Thus, preferred uses of such equipmentwill include appropriate ventilation systems, which can themselvesbecome quite elaborate and quite expensive.

A further, and perhaps the most significant, disadvantage of dryelectrode electrosurgical tools is revealed during cardiac ablationprocedures. During such a procedure, an electrode that is otherwiseinsulated but having an exposed, current carrying tip is inserted intothe heart chamber and brought into contact with the inner or endocardialside of the heart wall where the ablation is to occur. The current isinitiated and passes from the current generator to the needle tipelectrode and from there into the tissue so that a lesion is created.Typically, however, the lesion created by a single insertion isinsufficient to cure the irregular heartbeat because the lesion createdis of an insufficient size to destroy the errant electrical pathway.Thus, multiple needle insertions and multiple current applications arealmost always required to ablate the errant cardiac pathway, prolongingthe surgery and thus increasing the potential risk to the patient.

This foregoing problem is also present in TUNA procedures, whichsimilarly require multiple insertions of the needle electrode into theprostate gland. Failing to do so will result in the failure to create alesion of sufficient size otherwise required for a beneficial results.As with radiofrequency catheter ablation of cardiac tissue, then, theability to create a lesion of the necessary size to alleviate BPHsymptoms is limited and thus requires multiple insertions of theelectrode into the prostate.

A typical lesion created with a dry electrode using RF current and asingle insertion will normally not exceed one centimeter in diameter.This small size—often too small to be of much or any therapeuticbenefit—stems from the fact that the tissue surrounding the needleelectrode tends to desiccate as the temperature of the tissue increases,leading to the creation of a high resistance to the further passage ofcurrent from the needle electrode into the tissue, all as previouslynoted with regard to the formation of coagulum on an electrosurgicalscalpel. This high resistance—more properly termed impedance sincetypically an alternating current is being used—between the needleelectrode and the base electrode is commonly measured by the RF currentgenerator. When the measured impedance reaches a pre-determined level,the generator will discontinue current generation. Discontinuance of theablation procedure under these circumstances is necessary to avoidinjury to the patient.

Thus, a typical procedure with a dry electrode may involve placing theneedle electrode at a first desired location; energizing the electrodeto ablate the tissue; continue applying current until the generatormeasures a high impedance and shuts down; moving the needle to a newlocation closely adjacent to the first location; and applying currentagain to the tissue through the needle electrode. This cycle ofelectrode placement, electrode energization, generator shut down,electrode re-emplacement, and electrode re-energization, will becontinued until a lesion of the desired size has been created. As noted,this increases the length of the procedure for the patient.Additionally, multiple insertions increases the risk of at least one ofthe placements being in the wrong location and, consequently, the riskthat healthy tissue may be undesirably affected while diseased tissuemay be left untreated. The traditional RF ablation procedure of using adry ablation therefore includes several patient risk factors that bothpatient and physician would prefer to reduce or eliminate.

The therapeutic advantages of RF current could be increased if a largerlesion could be created safely with a single positioning of thecurrent-supplying electrode. A single positioning would allow theprocedure to be carried out more expeditiously and more efficiently,reducing the time involved in the procedure. Larger lesions can becreated in at least two ways. First, simply continuing to apply currentto the patient with sufficiently increasing voltage to overcome theimpedance rises will create a larger lesion, though almost always withundesirable results to the patient. Second, a larger lesion can becreated if the current density, that is, the applied electrical energy,could be spread more efficiently throughout a larger volume of tissue.Spreading the current density over a larger tissue volume wouldcorrespondingly cause a larger volume of tissue to heat in the firstinstance. That is, by spreading the applied power throughout a largertissue volume, the tissue would heat more uniformly over a largervolume, which would help to reduce the likelihood of generator shutdowndue to high impedance conditions. The applied power, then, will causethe larger volume of tissue to be ablated safely, efficiently, andquickly.

Research conducted under the auspices of the assignee of the presentinvention has focused on spreading the current density throughout alarger tissue volume through the creation, maintenance, and control of avirtual electrode within or adjacent to the tissue to be ablated. Avirtual electrode can be created by the introduction of a conductivefluid, such as isotonic or hypertonic saline, into or onto the tissue tobe ablated. The conductive fluid will facilitate the spread of thecurrent density substantially equally throughout the extent of the flowof the conductive fluid, thus creating an electrode—a virtualelectrode—substantially equal in extent to the size of the deliveredconductive fluid. RF current can then be passed through the virtualelectrode into the tissue.

A virtual electrode can be substantially larger in volume than theneedle tip electrode typically used in RF interstitial ablationprocedures and thus can create a larger lesion than can a dry, needletip electrode. That is, the virtual electrode spreads or conducts the RFcurrent density outward from the RF current source—such as a currentcarrying needle, forceps or other current delivery device—into or onto alarger volume of tissue than is possible with instruments that rely onthe use of a dry electrode. Stated otherwise, the creation of thevirtual electrode enables the current to flow with reduced resistance orimpedance throughout a larger volume of tissue, thus spreading theresistive heating created by the current flow through a larger volume oftissue and thereby creating a larger lesion than could otherwise becreated with a dry electrode.

While the efficacy of RF current ablation techniques using a virtualelectrode has been demonstrated in several studies, the currentlyavailable instruments useful in such procedures lags behind the researchinto and development of hoped-for useful treatment modalities for theablation of soft tissue and malignancies.

It would be desirable to have an apparatus capable of creating a virtualelectrode for the controlled application of tissue ablating RF electriccurrent to a tissue of interest so as to produce a lesion of desiredsize and configuration.

SUMMARY OF THE INVENTION

One aspect of the present invention provides a surgical apparatus forcreating a virtual electrode to ablate bodily tissue. The surgicalapparatus comprises an outer tube, a first electrode, an inner tube anda second electrode. The outer tube is fluidly connected to a source ofconductive fluid and defines a proximal end and a distal end. In thisregard, the distal end of the outer tube includes an opening fordelivering a conductive fluid from the outer tube. The first electrodeis disposed at the distal end of the outer tube and is configured toapply a current to conductive fluid delivered from the outer tube. Theinner tube is coaxially received within the outer tube. The inner tubeis fluidly connected to a source of conductive fluid and defines aproximal end and a distal end. In this regard, the distal end of theinner tube forms an opening for delivering a conductive fluid from theinner tube. The second electrode is disposed at the distal end of theinner tube. The second electrode is configured to apply a current toconductive fluid delivered from the inner tube. Upon final assembly, thedistal end of the outer tube is axially spaced from the distal end ofthe inner tube such that the first electrode is spaced from the secondelectrode. With this configuration, then, a bi-polar virtual electrodecan be established.

Another aspect of the present invention provides a surgical system forcreating a virtual electrode to ablate bodily tissue. The surgicalsystem includes a fluid source, a current source, and a surgicalinstrument. The fluid source maintains a supply of conductive fluid. Thecurrent source is configured to selectively supply an electricalcurrent. The surgical instrument includes an outer tube, a firstelectrode, an inner tube and a second electrode. The outer tube isfluidly connected to the fluid source and defines a proximal end and adistal end. The distal end of the outer tube includes an opening fordelivering the conductive fluid. The first electrode is disposed at thedistal end of the outer tube and is electrically connected to thecurrent source. The inner tube is coaxially received within the outertube and is fluidly connected to the fluid source. The inner tubedefines a proximal end and a distal end, with the distal end of theinner tube forming an opening for delivering the conductive fluid. Thesecond electrode is disposed at the distal end of the inner tube and iselectrically connected to the current source. Upon final assembly, thedistal end of the outer tube is spaced from the distal end of the innertube such that the conductive fluid is delivered as a first bolus fromthe outer tube and a second bolus from the inner tube. Current isapplied to the first and second boluses by the first and secondelectrodes, respectively. With this configuration, a bi-polar virtualelectrode can be created.

Another aspect of the present invention relates to a method for ablatingbodily tissue at a target site. The method includes delivering a firstbolus of a conductive fluid at the target site. A second bolus ofconductive fluid is also delivered at the target site, the second bolusbeing spaced from the first bolus. Finally, a current is substantiallysimultaneously applied to each of the first bolus and the second bolusto create a virtual electrode, ablating tissue in contact with the firstand second boluses. In one preferred embodiment, tissue between thefirst and second boluses is collapsed prior to applying the current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system for creating a virtual electrodein accordance with the present invention;

FIG. 2 is a perspective view, with a portion cut away, of a surgicalapparatus in accordance with the present invention; and

FIG. 3 is a schematic view of a portion of the surgical apparatus ofFIG. 2 ablating bodily tissue.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates in block form a system 10 for RF ablation useful withthe present invention. The system 10 includes a current source ofradiofrequency alternating electric current 12, a fluid source of RFablating fluid 14, including but not limited to saline and otherconductive solutions, and a surgical instrument 16 for delivering RFcurrent and ablation fluid to a tissue site (not shown) for ablationpurposes. In one preferred embodiment, the surgical instrument 16 isconnected to the current source 12 and the fluid source 14. It will beunderstood that the current source 12 and the fluid source 14 may becombined into a single operational structure controlled by anappropriate microprocessor for a controlled delivery of ablating fluidand a controlled application of RF current, both based upon measuredparameters such as but not limited to, flow rate, tissue temperature atthe ablation site and at areas surrounding the ablation site, impedance,the rate of change of the impedance, the detection of arcing between thesurgical instrument and the tissue, the time period during which theablation procedure has been operating, and additional factors asdesired.

While the surgical instrument 16 is shown as being connected to both thecurrent source 12 and the fluid source 14, the present system is not solimited but could include separate needles or other instruments usefulin RF liquid ablation procedures, that is, for example, a singlestraight or coiled needle having an exposed end and a fluid flow paththere through could be used to deliver both fluid and current to thetarget tissue for ablation purposes. Alternatively, a separate needlecould be used to deliver the current and a separate needle or needlescould be used to deliver fluid to the target tissue. In addition, theapplication of the present system is not limited to the use of straightneedles or helical needles as surgical instruments but could find usewith any type of instrument wherein a conductive solution is deliveredto a tissue and an RF current is applied to the tissue through theconductive fluid. Such instruments thus would include straight needles,helical needles, forceps, roller balls, instruments for the treatment ofvascular disorders, and any other instrument.

In one preferred embodiment, the system 10 further includes a secondfluid source 18 for delivery of tissue protecting fluid, via a deliveryinstrument 20, to a tissue whose ablation is not desired.

The surgical instrument 16 may assume a wide variety of forms. Onepreferred embodiment of a surgical apparatus 50 is shown in FIG. 2. Theapparatus 50 generally includes a bi-polar electrode that is useful inan ablation procedure using an RF ablating fluid such as, but notlimited to, isotonic or hypertonic saline. Further, the apparatus 50generally includes a plurality of coaxial thin walled tubes forming acatheter delivery system for RF ablating fluid and RF ablating current.Thus, the apparatus 50 preferably includes a first or outer thin walledtube 52 having a proximally attached hemostasis valve 54. The valve 54includes an inlet 56 for RF ablating fluid flow as indicated by arrow 58and also an access port 60 for an electrical line 62, which can beelectrically connected to the RF current source 12 (FIG. 1). A distalend of the first tube 52 may be provided with one or more thermocouplesor other temperature sensors 64. The distal end of the first tube 52also includes an electrode 66 which is electrically connected to theline 62, thereby providing one of the two bi-polar electrodes envisionedby a preferred embodiment of the present invention. It will beunderstood that the first tube 52 is electrically insulated or otherwisenon-conductive. The first tube 52 provides a flow passage for the RFablating fluid from the fluid source 14 (FIG. 1) to the distal end ofthe first tube 52 where it exits the first tube 52 as indicated byarrows 68.

The apparatus 50 further includes a second or intermediate thin walledtube 70 coaxially disposed within the first tube 52. The second tube 70has a proximally attached hemostasis valve 72. The valve 72 includes asuction port 74 for directing fluid flow as indicated by arrow 76. Adistal end of the second tube 70 may be provided with one or more vacuumapertures 78 through which a suction is applied to surrounding tissue(not shown), via a vacuum formed at the suction port 74. Thus, applyingsuction to the suction port 74 will draw the surrounding tissue intocontact with the vacuum apertures 78 and the distal end of the secondtube 70. In addition, with this same process, some RF ablating fluid maybe removed via the applied suction as indicated by arrows 80.

The apparatus 50 further includes a third or inner thin walled tube 82coaxially disposed within the second tube 70. The third tube 82 has aproximally attached hemostasis valve 84. The valve 84 includes an inlet84 for RF ablating fluid flow as indicated by arrow 86 and also anaccess port 88 for an electrical line 90, which can be electricallyconnected to the current source 12 (FIG. 1). A distal end of the thirdtube 82 also includes an electrode 92, which is electrically connectedto the line 90, thereby providing the other one of the two bi-polarelectrodes envisioned by a preferred embodiment of the presentinvention. It will be understood that the third tube 82 is electricallyinsulated or otherwise non-conductive. THE third tube 82 provides a flowpassage for the RF ablating fluid from the fluid source 14 (FIG. 1) tothe distal end of the third tube 82 where it exits the third tube 82 asindicated by arrows 94.

The surgical apparatus 50 may further include a probe 100 that extendsthrough an interior passage of the third tube 82 and that is freelymovable therewithin. The probe 100 may include a thermocouple 102disposed at a most distal end thereof and may be connected via anelectrical line 104 to the RF current source 12 (FIG. 1) to provide atemperature measurement at a predetermined distance from the electrodes66 and 92.

In operation, following delivery to a particular target site, RFablating fluid will be provided to the ports 56 and 84 from the fluidsource 14 (FIG. 1). The fluid will exit the distal ends of the firsttube 52 and the third tube 82, respectively, and begin to form bolusesof fluid along or within the tissue at the target site. Application of asuction at the vacuum aperture 78, via the suction port 74, will causethe tissue surrounding the distal end of the second tube 70 to becollapsed or pulled into a substantially fluid tight relationship withthe second tube 70, thereby preventing migration of the fluid, inparticular via a capillary effect, along the second tube 70 between thedistal ends of the first and third tubes 52, 82. RF ablating power canbe applied and an ablation procedure can be carried out. By theapplication of suction to the tissue and the prevention of the flow offluid along the second tube 70, shorting of the current between theelectrodes 66 and 92 can be avoided.

FIG. 3 represents schematically the fluid flow and current flow achievedduring use of the present invention. Thus, FIG. 3 shows a tissue 110,such as liver, into which a portion of the surgical apparatus 50 hasbeen inserted. Infusion of the RF ablating fluid will initially createtwo separate boluses of fluid, 112 and 114. If not for the suctionapplied to the tissue surrounding vacuum apertures 78, fluid would tendto travel along the path created during insertion and placement of theapparatus 50 in the tissue 110. Applying suction intermediate therelease of the fluid, however, prevents such fluid travel andsubstantially prevents any shortage between the electrodes 66, 92directly therebetween. The applied current can thus spread readilythrough the boluses 112, 114 and then travel therebetween as indicatedby the lines 116. In this way, then, ablation can be accomplishedbetween the bi-polar electrodes 66 and 92. The lesion created with theuse of such a bi-polar structure will essentially be the size of theboluses 112, 114 at a minimum, though it will be understood thatcell-to-cell thermal conduction will occur that will make the lesion inreality larger than the boluses 112, 114, assuming that power is left onfor the appropriate length of time.

As is well known, when an electro-surgical tool is used, or for thatmatter, any electrical device, a complete circuit for current flow mustbe provided. Thus, when a monopolar surgical instrument such as a bovieknife is used, a ground pad connected to the RF current generator isplaced under the patient to provide a complete electrical circuit fromthe generator, to the knife, through the patient to the ground pad, andback to the generator. With the present invention, no ground pad isneeded. The current flows from the current source 12 (FIG. 1) to one ofthe two electrodes 66 or 92 on the apparatus 50, through the patient tothe other electrode 66 or 92, and then back to the current source 12.Such bi-polar structure enables the physician to localize the effect ofthe current passing through the patient, thereby avoiding certain risksassociated with the use of a monopolar device, including but not limitedto the risk of burns where the ground pad contacts the patient.Additionally, by controlling the relative spacing between the twoelectrodes 66, 92 of a bi-polar instrument, the size of the lesion canbe controlled. Thus, where the electrodes 66, 92 are close to eachother, there will be little tissue between them through which thecurrent will travel and the lesion size will be reduced relative to aninstrument where the electrodes are spaced relatively farther apart. Inaddition, the shape of the lesion is somewhat controllable bycontrolling both the fluid flow and the spacing of the electrodes 66,92. It will be understood that regardless of the device used that thelesion size and shape is a product of many factors, including the tissuecomposition, the conductivity of the fluid, the amount of appliedcurrent, the amount of cell-to-cell thermal conductivity, the timeperiod the current is applied, tissue temperature, and fluid temperaturewhen applied, among others.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A surgical apparatus for creating a virtual electrode to ablatebodily tissue, the apparatus comprising: an outer tube fluidly connectedto a source of conductive fluid, the other tube defining a proximal endand a distal end, the distal end of the outer tube including an openingfor delivering a conductive fluid from the outer tube; a first electrodedisposed at the distal end of the outer tube for applying a current toconductive fluid delivered from the outer tube; an inner tube coaxiallyreceived within the outer tube, the inner tube being fluidly connectedto a source of conductive fluid and defining a proximal end and a distalend, the distal end of the inner tube forming an opening for deliveringa conductive fluid from the inner tube; and a second electrode disposedat the distal end of the inner tube for applying a current to conductivefluid delivered from the inner tube; wherein upon final assembly, thedistal end of the outer tube is axially spaced from the distal end ofthe inner tube such that the first electrode is spaced from the secondelectrode.
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